| Bentonite has been widely studied in dye and heavy metal wastewaters treatment due to its characteristics of the ability of ion exchange and high specific surface area. Methods for modified bentonite have been widely studied but modifying bentonite with organic acid directly has never been reported, therefore, in the study, citric acid was selected as a modified agent from the standpoint of its cheap and environment friendly feature. Citric acid bentonite (CAB) was obtained by modifying RB with citric acid directly and its adsorption mechanisms in dye wastewater were studied.In this paper, the optimum conditions of CAB preparation were presented and the mechanisms of modified CAB were also studied. CAB was used in the adsorption of anionic dye, Congo red (CR), and cationic dye, Methylene blue (MB) from aqueous solution. The effect of adsorbents dosage, contact time, pH, salts, surfactants, initial concentrations and temperature of dyes were investigated in detail. The adsorption mechanisms of CR and MB on CAB were raised basing on the studies of adsorption isotherms, kinetic models and materials characterization. The main results are as follows:(1) The optimum preparation process of CAB was that, the ratio of citric acid to bentonite was180%, the reaction time was2h and the modified temperature was60℃. RB modification with citric acid was a surface modification. After modification, the specific surface area (SBET) of CAB decreased from40.1to16.4m2·g-1, and the average pore diameter increased from8.06to11.7nm.(2) The effects of adsorbents dosage, contact time, pH, salts, surfactants, initial concentrations and temperature of dyes on CAB adsorption were investigated. It was found that pH had a great effect on CR removal and it had little effect on MB removal. CR removal on CAB could be promoted in the presence of NaCl and CaCl2, and the removal effect was more obvious in the presence of CaCl2than that of NaCl. We proposed there were two reasons for this phenomenon:one was Ca2+carries more positive charge than Na+which could neutralize more negative charged CR molecules and the other reason was that Ca-CR precipitation was formed in the presence of Ca2+in CR solution which could promote CR removal. MB removal efficiencies on CAB were restrained in the presence of salts. CR removal on CAB increased firstly and decreased afterwards in the presence of cationic surfactant of CTAB and that decreased at first and then increased steadily in the presence of anionic surfactant of SDBS. MB removal on CAB was restrained in the presence of CTAB and that changed little in the presence of SDBS.(3) CR and MB adsorption on CAB could be described by R-P model and Langmuir model and the respective Langmuir adsorption capacity was384and252mg-g"1at303K. The D-R model showed CR and MB adsorption on CAB was held by chemisorption. The kinetic models showed that adsorption of CR and MB on CAB followed the pseudo-second order model. The intra-particle diffusion was also involved in the process at the same time.(4) The adsorption mechanisms of dyes on CAB were proposed as: CR adsorption on CAB is a external surface adsorption process and in the process, there exist electrostatic forces between CAB and the groups of CR, there also may be exists CR/CAB complexation in the adsorption process. The groups of carboxyl and hydroxyl are the major adsorption sites for MB removal.In summary, the preparation of CAB was simple and CAB performed well in dyes removal. This study showed CAB is a promising adsorbent in dyes wastewater treatment. |
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